Neutron (quasi-elastic) scattering experiments have been carried out to investigate the effects of temperature and pressure on the motion of molecules in liquid H2O. Spectra were obtained at several temperatures between −10 and +24 °C at atmospheric pressure, and also at +1 °Cand 1.42 × 108 Pa. The root-mean-square amplitude of proton motion in solid and liquid H2O at atmospheric pressure has been inferred from measurements of the Debye–Waller factor and is nearly constant at 0.07 nm for both the solid and liquid. A model in which the translational diffusion and rotational diffusion are based on Brownian motion was fitted to our data using published diffusion coefficient data. The structure breaking time was found to be very small at 1 and 10 °C. Both these conclusions are compared with those reported in several earlier papers, which gave varying results.The spectrum measured at 1.42 × 108 Pa was slightly broader than the corresponding spectrum at atmospheric pressure, probably indicating that the well-known minimum in the viscosity at this pressure is due to a structure breaking effect.

The structure in the photoneutron cross sections of 16O and 208Pb has been studied by the measurement of high resolution photoneutron energy spectra using the neutron time-of-flight technique and bremsstrahlung irradiations. For 16O, the ground state differential cross section at 98° has been deduced between 17.3 and 28.5 MeV and is in good agreement with most previous studies. Fine structure is seen throughout the cross section. Eight neutron energy spectra for 208Pb from bremsstrahlung endpoint energies in the range 11.0 to 15.5 MeV were obtained. Strong peaks are seen at center of mass neutron energies of 1.67, 1.85, 2.06, 2.19, 2.68, 3.15, 3.27, 3.50, 3.77, and 4.03 MeV with weaker peaks elsewhere. The energies of these peaks are in good agreement with previous measurements in this laboratory. The energies of peaks in the spectra are compared with recent cross section measurements.

Experiments to measure the photoneutron yield from a liquid $^{4}\mathrm{He}$ target as a function of beam intensity have been carried out. They show that the most recent measurements of the photoneutron cross section with liquid helium targets can be made consistent with each other, and that the results are then not inconsistent with measurements of the $^{4}\mathrm{He}(\ensuremath{\gamma},n)^{3}\mathrm{He}$ cross section using gaseous targets.[NUCLEAR REACTIONS $^{4}\mathrm{He}(\ensuremath{\gamma},n)$, $E=35$ MeV; measured $\ensuremath{\sigma}$ (beam current).]

The 98 deg differential photoneutron cross section for /sub 4/He has been measured for excitation energies between 22 and 33 MeV with a liquid /sup 4/ He target at its normal boiling point and for excitation energies between 23 and 37 MeV with a /sup 4/He gas target at a pressure of 5l.6 bars. The cross section values obtained with the gas target were approximately a factor of 1.9 greater than those obtained with the liquid target. This apparent dependence of the cross-section upon the physical state of the /sup 4/He target may explan the large values for the ratio sigma ( gamma , p)/ sigma ( gamma , n) obtained by comparing photoneutron and photoproton cross sections obtained with targets in different physical states. (auth)

The 98 \ifmmode^\circ\else\textdegree\fi{} differential photoneutron cross section for $^{4}\mathrm{He}$ has been measured for excitation energies between 22 and 33 MeV with a liquid $^{4}\mathrm{He}$ target at its normal boiling point and for excitation energies between 23 and 37 MeV with a $^{4}\mathrm{He}$ gas target at a pressure of 51.6 bars. The cross-section values obtained with the gas target were approximately a factor of 1.9 greater than those obtained with the liquid target. This apparent dependence of the cross section upon the physical state of the $^{4}\mathrm{He}$ target may explain the large values for the ratio $\frac{\ensuremath{\sigma}(\ensuremath{\gamma},p)}{\ensuremath{\sigma}(\ensuremath{\gamma},n)}$ obtained by comparing photoneutron and photoproton cross sections obtained with targets in different physical states.

The spectra of deexcitation gamma rays following photodisintegration of 19F and 31P have been measured to study further the reaction channels involved in photodisintegration of nonclosed shell nuclei. Besides (γ,n) and (γ,p) decays to excited states of the residual nuclei, strong (γ,α) channels from 19F to the first excited state of 15N and from 31P to the first two excited states of 27Al were observed.

A facility for measuring the angular distributions of photo-protons form solid targets is described. The proton spectrometer consists of an evacuated target chamber with lithium-drifted silicon detectors at angles of 30°, 60°, 90°, 120° and 150° to the direction of the photon beam. When the proton spectrometer is used in conjunction with a neutron detector (an 18 in. × 3 in. × 2 in. slab of plastic scintillator viewed by two photomultiplier tubes, one at each end), the facility becomes capable of detecting neutrons and protons in coincidence. These can be detected over a wide range of angles with a resolving time of ≦ 50 nsec.